Radar level gauging (RLG) is an increasingly important method for level gauging in tanks, containers, etc. A radar level gauge typically includes transceiver circuitry and processing circuitry which is connected to a propagation device adapted to allow propagation of electromagnetic signals into the tank. An example of such a systems is disclosed in U.S. Pat. No. 7,106,247 by the same applicant.
The signal propagating device may be a directional antenna. However, in some applications radar level gauges using antennas to provide free propagating signals are not suitable, and a wave guiding structure is used to guide the waves. Based on propagation mechanism three different types of wave guiding structures are known in the prior art.
Wave guides of the first type are hollow (e.g. a pipe of suitable cross section) and “thick” in the sense that they have a cross section of half a wavelength or more, possibly reduced by a dielectric filling. The electromagnetic fields in such a waveguide always have at least one field component along the direction of propagation. When used in radar level applications, wave guides of this type are referred to as “still pipes”, and must be perforated to get the same liquid level inside as outside.
Wave guiding structures of the second type are transmission lines with two or more conductors, such as a twin line or a coaxial line. Transmission line wave guides have a diameter much smaller than the wavelength of the transmitted waves, and one typical feature is that the electromagnetic fields are transverse or of TEM-type (Transverse Electro-Magnetic fields). A twin line with two stainless wires and sparse dielectric separations is one practical example. For practical level gauging applications using signals below 1 GHz (with wavelength above 300 mm) a transmission line diameter (or maximum transversal measure) of 3-20 mm is commonly used. A too small diameter will increase resistive losses and may cause problems with material clogging and mechanical strength.
Finally, wave guiding structures of the third type are surface wave guides (SWGs), such as a single wire transmission line or tube with or without dielectric coating. A surface wave guide can be very thin as compared to the wavelength (4-8 mm is a common SWG diameter for use below 1 GHz) but they also have field along the propagation direction and also fields well outside of the SWG. In contrast to the transmission line probe of TEM type it needs more space free as there are fields more distant from the wire. In case of a single metal wire, a poor electric conductor such as stainless steel is suitable. The single wire probe is very practical and robust to use for level gauging and has lower attenuation per meter than for example a twin wire made of the same type of conductor.
Wave guiding structures of the second and third type thus both have a diameter much smaller than the wavelength of the transmitted waves. In radar level gauging applications such waveguides are normally referred to as “probes”, and the detection principle is sometimes referred to as guided wave radar (GWR). The most common type today uses short pulses (around 1 ns) without carrier and occupy a frequency range of roughly 0.1-1 GHz.
Due to safety restrictions, environmental laws and requirements, etc, there is often a need for several measurements of the surface level of the medium in the container, which measurements are completely separated and functionally independent from each other. For example, in radar level gauging systems for a tanker's load containers, at least one alarm function (e.g. overfill alarm) that is functionally independent of the level measuring system is required.
Functional independence not only requires separate units but also means that a fault in one system does not render the other system(s) to be degraded or inoperative. Such independence can be achieved by ensuring that there are no common electrical circuits and cabling, i.e. there must not be any galvanic contact between different measuring systems. One way to achieve this is to simply install at least two complete independent level gauges.
However, in recent years, the cost for the mechanical installation of a radar level gauge has increased faster than the cost for the electronics in the gauge. To let two or more electronics units use the same signal propagating device, such as an antenna, is therefore cost efficient and advantageous, provided that the common parts can be assumed to have very high reliability. Such systems have shown to be successful on the market, especially as various safety regulations often require redundancy in critical functions. When some parts (like antenna or a still pipe) are shared by two level gauges the design must be done to maintain sufficient electrical isolation between the two gauges. If the faulty level gauge should generate unusual signals or present unusual impedance the circuits should be done to isolate the correct gauge from disturbances which might degrade the other unit outside of its specification.
The prior art, illustrated for example by U.S. Pat. No. 6,414,625, 6,765,524 and US 2013/0009803, typically relates to systems where a plurality of gauges share a common signal propagation device, and provide two or three different functions, e.g. level gauging and high level alarm. On the signal side of the gauges, a power supply and signal interface, typically provided together by two wires (a so called bus), may be shared by the gauges or be provided separately (the latter case is often referred to as “galvanically separated” installations). In case of a standardized bus, the connection is specified to be such that an arbitrary error in one unit does not interfere with the connection of the other. The type testing procedure for the bus connection among other ensures that such a failure is very unlikely.
On the radar side of the gauges, there is a similar requirement that a dysfunctional gauge must not interfere with another gauge. This requirement has so far in practice limited the available solutions for connecting several gauges to a single propagating device to solutions involving different and distinguishable signals or propagation modes. For example, patents '625 and '524 (mentioned above) disclose two or more radar channels connected to an antenna, where the signals are distinguishable e.g. by having different polarization modes. The more recent US2013/0009803 (also mentioned above) discloses connecting two electronics units to a GWR probe with different propagation modes (e.g. a TEM mode and a surface wave guide mode or more than one TEM-mode in a multi-conductor probe). In both these cases, the isolation between the different modes can relatively easily be ensured to exceed the required approximately 20 dB. With around 20 dB or more suppression, typical level gauging signal processing must be able to neglect disturbances. The typical echo signal for a radar level gauge sometimes contains disturbances of that order. Thus any typical signal processing has to handle disturbances of that order.
In summary, the prior art suggests using a propagation device (antenna, hollow waveguide or multimode transmission line) allowing more than one propagation mode (or polarization) and corresponding coupling structure to provide sufficient isolation between the two or three independent level gauging functions. If a single wire probe is used there is only one propagation mode available and prior art solutions cannot be used. It is possible to separate signals having distinguishable signal features (like stated in '625 above) but it is not possible to surely know the signals from an electronic unit dysfunctional in an unspecified way. In the last case measures have to be taken to ensure sufficient isolation between a faulty unit and a correct one.
So far, there are no known solutions which allow connection of multiple gauges to a single wire transmission line probe with sufficient isolation between the gauges to make the units sufficiently independent if abnormal function should occur in one unit. The methods used in some bus connections on the signal side involve big signal attenuation and the small power margins do not allow them to be copied on the radar side.
General Disclosure of the Invention
It is an object of the present invention to provide a multi-channel level gauge using a single wire transmission line probe.
This and other objects are achieved by a level gauge comprising a first circuitry arrangement with first transceiver circuitry for transmitting first electromagnetic transmit signals and receiving first electromagnetic echo signals, and first processing circuitry connected to the first transceiver circuitry for determining a first process variable, and a second circuitry arrangement with second transceiver circuitry for transmitting second electromagnetic transmit signals and receiving second electromagnetic echo signals, and second processing circuitry connected to the second transceiver circuitry for determining a second process variable. A power divider is electrically connected to the first transceiver circuitry and to the second transceiver circuitry to provide signal isolation between the first transceiver circuitry and the second transceiver circuitry. A process seal provides a sealed electrical feed-through from the power divider through a tank wall, the electrical feed-through having an electrically matched connection with the power divider. The gauge further comprises a single wire transmission line probe mechanically suspended by the process seal and extending into the content in the tank, the single wire transmission line probe being adapted to guide the transmit signals towards and into the content, and to guide reflected signals back to the first and second circuitry arrangements. The electrical feed-through has a first input impedance as seen from the probe, and the single wire transmission line probe has a second input impedance as seen from the electrical feed-through, and a matching arrangement is arranged to provide an electrically matched connection between the electrical feed-through and the single wire transmission line probe.
It is noted that the term power “divider” is used, although its function is to act as a gateway between the two circuitry arrangements and the one single probe, while maintaining electrical isolation between the two circuitry arrangements and the first and second transmit signals.
It is also noted that both transmit signals, after passing the power divider, are connected to one single electrical feed-through in the tank wall.
The single wire transmission line probe is typically a surface wave guide along which signals can propagate only in one and the same propagation mode.
The power divider essentially ensures sufficient isolation between the two transceivers and the two circuitry arrangements (sometimes referred to as two “channels”). However, in the case of a single wire transmission line probe, the normally poor matching (typically a reflection factor of −3 dB) would deteriorate the isolation. The solution according to the present invention therefore also includes a matching arrangement between the probe and the electrical feed-through. Such a matching arrangement is known per se, and has been suggested in other applications. However, the present invention is based on the realization that a combination of a power divider with a matching arrangement is necessary to allow multiple channels on one single wire transmission line probe, as the isolation of the power divider otherwise would be destroyed by the very strong mismatch reflection (−3 dB when the single wire probe is connected to 50 ohm) where the single wire probe is connected.
In many other radar level gauging applications a power divider such as a Wilkinson power divider (essentially a stepped impedance transformer) could be directly matched to the impedance of the single line, but for the guided wave radar level gauging application the sealed suspension of the probe typically has relatively low impedance. The relationship between the impedance of the probe in free space and the impedance of the electrical feed-through (facing the tank) may be four times, five times, six times, or even more. For example, the impedance of the electrical connection through the process seal may be less than 50 ohm or even less than 40 ohm, while the impedance of the probe in free space in the tank may be more than 200 ohm, or even more than 350 ohm. As an example, a non-coated single wire probe with a diameter of 4-10 mm has an impedance in the range 300-400 ohm in free space for relevant operating frequencies.
According to preferred embodiments, the signal isolation provided by the power divider is such that said first and second circuitry arrangements are functionally independent. This means that the isolation is sufficient to allow specified function for one of the gauges under any type of failure on the other gauge(s). As mentioned in the background, the two gauges may be part of different systems (e.g. one system for level gauging and one system for high level supervision) with totally independent use. In preferred embodiments, the isolation is at least 15 dB, or at least 20 dB.
The provision of sufficient isolation between the circuitry arrangements ensures little or no leakage between the channels. As a consequence, it is not necessary that the first and second electromagnetic signals are distinguishable from each other. In fact, according to a preferred embodiment, the first and second electronics units are designed to transmit substantially identical electromagnetic signals, i.e. signals having the same frequency and amplitude behavior. It is noted that the two circuitry arrangements typically are not used exactly simultaneously. However, the design according to the invention intends to minimize mutual interference. Even if the two arrangements are used in different overall systems (such as level measurement and overfill detection) interference needs to be avoided.
In this case the circuitry arrangements may be functionally identical, and most preferably multiple samples from the same manufacturing process. This makes the system efficient to manufacture. Two substantially identical units are simply connected to the power divider, which is connected to the single line transmission line probe via the matching arrangement.
Each circuitry arrangement preferably includes a power limiting power supply interface for receiving electrical power. This interface can provide drive power to the first and second circuitry arrangements in such a way that intrinsically safe operation is ensured. By thus restricting the power available to each circuitry arrangement, the potential risks of cross-talk is further reduced. Even if one circuitry arrangement provides an erroneous signal which resembles a surface echo, this signal will be limited to a very short distance range and to cases where a very weak signal is measured by the correct transmitter.
According to one embodiment, the circuitry arrangements have at least one variable operation parameter. Such a variable operation parameter may be changed at pre-scheduled points in time, or be changed in response to control signals received over a signal interface. By changing a variable operational parameter, such as the pulse repetition frequency in a pulsed radar level gauge, it is possible to eliminate interfering signals from a non-functional circuitry arrangement even in rare cases where the 20 dB isolation may not be quite sufficient.